Particles of various types are commonly incorporated into rubber compositions as fillers. For instance fillers are typically included in the rubber compositions utilized in manufacturing tires, hoses, belts, windshield wiper blades, floor mats, and other rubber products. The fillers are chosen to enhance certain physical characteristics of the rubber composition. The use of carbon black, silica, and crosslinked polymers as fillers is well known in the art of rubber compounding. The addition of fillers tends to improve the physical properties of the polymer matrices to which they are added. Some specific physical properties that are improved by the addition of fillers are tensile strength and abrasion resistance. For instance, the inclusion of fillers in tire tread rubber compounds is critical to attain required strength and tread wear characteristics.
The use of rigid polymer particles to enhance the properties of rubbery materials has been explored by numerous researchers. Early work by Kraus et al, “Dynamic Properties of a Model Reinforced Elastomer. Styrene-Butadiene Reinforced with Polystyrene” in Macromolecules, Vol. 3, Issue 1, pages 92-96 (1970) studied the properties of 40 nm polystyrene particles reinforced SBR rubber. Morton et al also examined similar systems in the article titled “Mechanism of Reinforcement of Elastomers by Polymeric Fillers” in Advanced Chemistry Series, No. 99, pages 490-509 (1971). Recently, Cai et al reported in the Journal of Materials Science, 34, pages 4719-4726 (1999) that crosslinked polystyrene particles in polysulfide rubber gave improved modulus, fracture strength and elongation at break. However, in all these studies, the particles had limited interaction at the interface between the particles and matrix which resulted in physical properties that were only marginally improved.
U.S. Pat. No. 5,395,891 and U.S. Pat. No. 6,127,488 to Obrecht disclose the use of crosslinked polybutadiene and crosslinked styrene-butadiene copolymers, respectively, as fillers. The purported benefits of the inclusion of these crosslinked rubber particles in rubber formulations is lower hysteresis while the polybutadiene gels also impart improved abrasion resistance and styrene-butadiene copolymer gels offer improve wet traction characteristics. U.S. Pat. No. 6,133,364, U.S. Pat. No. 6,207,757, and U.S. Pat. No. 6,242,534 to Obrecht et al disclose a rubber composition comprising a crosslinked elastomer particle and a non-crosslinked elastomer. The crosslinked elastomeric particles are functionalized to impart unique surface reactivity for improved coupling with the non-crosslinked host elastomer as well as the subsequent ability to employ coupling agents other than the conventional sulfur-bridged bis-alkoxy silanes. The benefits of these gels in tire tread compositions are reported to be lower hysteresis, improved abrasion resistance, and improved wet traction.
U.S. Pat. No. 6,747,095 and U.S. Pat. No. 6,653,404 to Konno et al disclose a rubber composition comprising as essential components a crosslinked polymer particle and a non-crosslinked rubber component. The rubber composition may be used to obtain a vulcanized rubber. The crosslinked polymer particles are intended for use as filler to provide a rubber composition having good processability and handling performance as well as improved tensile strength and wear resistance when vulcanized. However, it has been found that the particles disclosed in the Konno et al. application tend to soften at higher service temperatures. The effectiveness of the particles as reinforcing fillers is therefore decreased.
U.S. Pat. No. 6,437,050 to Krom et al disclosed a polymer nanoparticle composition wherein the core is a poly(alkenylbenzene) and the surface layer includes a poly(conjugated diene). The particle is a star polymer with all the diblock copolymer chains attached to one crosslinking center. An article from Fetter and Bi titled “Synthesis and Properties of Block Copolymers 3. Polystyrene Polydiene Star Block Copolymers” in Macromolecules, Vol. 9, Issue 5, pages 732-742 (1976) also studied and reported on the identical system. These particles do not have well-defined center particles. The polystyrene domains are formed from phase separation of diblock copolymers. It lacks dimension stability at higher service temperature above the glass transition temperature of polystyrene.
Nanoparticles are currently receiving significant interest as fillers in rubber compositions. Such combinations often provide dimensions and physical properties that differ from those of the bulk material. The use of nanoparticles as fillers is thought to be advantageous because the discrete particles may be more easily and better dispersed in the polymer matrix. The higher surface to volume ratio of the nanoparticles provides a better opportunity for chemical and physical interactions with the polymer matrix.